Absorption-type modulation system



Nov. 14, 1950 c. M. SINNETT 2,530,087

ABSORPTION-TYPE MODULATION SYSTEM Filed March 9, 1945 INVENTOR. CHE'JTiR M SIN/V577 ATTORNEY Patented Nov. 14, 1950 2,530,087 ABSORPTION-TYPE MODULATION SYSTEM Chester M. Sinnett, Westmont, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 9, 1945, Serial No. 581,903

8 Claims. (Cl. 179-100.4)

to a high frequency oscillator thereby to varythe amount of power absorption from the oscillator with the result that the oscillator space current varies correspondingly in amplitude and provides said direct current variations.

Another important object of my invention is to provide a phonograph record reproducer whose tone arm includes a capacity pickup device at its head; the pickup device being adapted to vary the effective length of a resonant line coupled thereto but havin no direct connection to it, and the resonant line functioning to vary the plate current of a high frequency oscillator tube.

Another object of my invention is to provide a phonograph record reproducer wherein the base or mounting for the tone arm functions as a mechanical support for a high frequency oscillator, a resonant line running through the tone arm from the pickup device to the oscillator, and reactive variations at the pickup causing corresponding power absorption variations by the resonant line.

A more specific object of my invention is to provide a record reproducer utilizing an oscillator operating above 100 megacycles (mc.) and comprising a concentric line resonator having a miniature type tube associated therewith; the

tone arm being provided with a capacitypickup device which is adapted to vary the effective electrical length of a half-Wave length line loosely coupled to the aforesaid concentric line resonator.

Still other objects Of my invention are to improve generally the efficiency and reliability of record reproducers, and more especially to provide high frequency record reproducing systems which are not only economical to manufacture and assemble, but are adapted to occupy a minimum of space.

Still other features will best be understood by reference to the fOllOWiIlg description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a circuit diagram of one embodiment of the invention; and

Fig. 2 shows mechanical details of a preferred embodiment of the invention, certain parts being sectioned. Referring now to the accompanying drawing, wherein like reference numerals in the difierent figures indicate similar elements, there is shown in Fig. l the circuit diagram of a system adapted to reproduce sound records of any suitable type. The usual tone arm, shown sectioned to reveal its interior, is pivoted at 2 to any suitable swivel joint. Those skilled in the art of phonograph construction are fully aware of the manner of constructing the tone arm, turntable and associated parts thereof. Accordingly, only so much of the mechanical elements are described herein as are essential to a proper understanding of my invention. At the pickup head 3 there is located a reactive element whose magnitude is varied in response to displacements of a stylus or needle 4. The reactive element specifically is a condenser C consisting of a mobile electrode 5 and a stator or fixed electrode 6. The stylus 4 is mechanically coupled to the mobile electrode 5 in any suitable manner. By way of example, capacity C may be provided by a condenser of the ribbon type disclosed and claimed in my U. S. patent application Serial No. 414,305, filed October 9 1941 now Patent No. 2,376,456 granted May 22, 1945. Indeed, the entire tone arm assembly could be constructed in the manner shown in Fig. 3 of this application.

The stylus 4 is adapted to scan or ride the sound-representative grooves of a sound record as the latter is rotated on a turntable (not shown). The vibration of the stylus 4 causes corresponding vibration of electrode 5 thereby to vary the capacity of condenser C in a corresponding manner. My invention is not limited to condenser C being a record pickup device, since the electrode 5 could be varied by the diaphragm of a microphone. Further, the mobile electrode 5 could be displaced in response to any physical change thereby to cause an electrical indication at a resistor H3 in an oscillator output circuit.

Inaccordance with my invention, the electrodes 5 and -6 are coupled to respective conductors of a resonant line of a predetermined electrical length. For example, line I, 1 has its input terminal connected to electrode 5, while line 8, 8 has its input terminal connected to electrode 6. The condenser 9 connects sections 1 and I, and condenser 10 connects sections 8 and 8'. The

sections or conductors I and 8 are run, preferably parallel, through the length of the tone arm, while conductors I and 8' are located outside the tone arm. The electrical length of the resonant line comprising the conductors I, 'I'8, 8 is approximately one-half a wavelength long at the operating oscillator frequency. The conductors could be provided by parallel lamp cords, if desired.

The function of condensers 9 and I9 is to eliminate the need for connecting wires between the internal and external sections of the resonant line. In this way the stiifness of connecting wires is avoided. As pickups of lower vertical force are used, it becomes necessar to provide less friction in all bearings. The elimination of connecting leads between the sections of the resonant line is a step in this direction. It is to be understood, however, that condensers 9 and I each have a relatively large capacity so that they will not interfere with the action of condenser C in varying the electrical length of the resonant line. At the high frequency employed in the oscillator, the reactance of each of condensers 9 and ID is low. The condensers 9 and I0 are schematically represented, and may be provided in any suitable manner. For example, the upper electrodes of the two condensers could be movable with the tone arm, while the lower electrodes could be fixed. In Fig, 2 is shown such a construction providing one of the two condensers. It will be noted that the tone arm I has no direct connection with the line sections I, 8, but is coupled thereto through the condensers 9 and Ill.

As the capacity of condenser C varies, the effective electrical length of the resonant line is varied correspondingly. In accordance with my invention, I utilize the variations in effective length of the resonant line to vary the space current of an oscillator tube. The oscillator tube II may be of any suitable type. I prefer to employ a miniature type of tube, say one of the 9002 type, whose cathode I2 is grounded. The control grid I3 of tube II is returned to ground through the grid leak resistor I4, while the plate or anode I5 is connected by conductor I6 and output resistor I8 to the +18 terminal of any suitable direct current source.

The resonant tank circuit of the oscillator is composed of conductor I6 and conductor I9 con- F nected to grid I3. The extreme end of conductor I9 is connected by tuning condenser 29 to the conductor I8, and a parallel condenser 2I shunts condenser 29 between the conductors I6 and I9. The condensers 20 and H provide regenerative coupling, or feedback, between the plate I5 and I3. The conductors I6 and I9 may be provided by a concentric line resonator, and the tank circuit is tuned to a high frequency which may be, for example, 200 me. In general, the oscillation frequency may be chosen from a range of 100 mc. to 200 mc., or higher. Condenser 29 may be dispensed with so far as the operation of the oscillator is concerned.

The oscillator system may, of course, be of any suitable construction commonly employed in the region above 100 mc. While the resonant line I, 'I'8, 8 is given an effective electrical length which is approximately half a wavelength at the oscillator frequency, the invention is not restricted to the specific electrical length since any other suitable electrical length, as for example wavelength, may be employed. There is a range of tolerance in this connection. Preferably the effective electrical length of the resonant line should not exceed :10% deviation from one-half a Wavelength. The physical length of the resonant line should be such as to conform to the ideals of compactness and economy. In accordance with my invention, the end portion of the conductor I is loosely coupled to the tank circuit of the oscillator. I have shown the conductor I positioned adjacent the ground conductor I9 of the tank circuit. The length of the portion of I adjacent conductor I9 is a matter of circuit design, and is not critical. The opposite conductor 8 need not be coupled to the tank circuit, and is illustrated as being positioned to be out of the oscillator field.

The variation of the capacity of condenser C changes the tuning of the resonant line I, 'I'8, 8. The natural frequency of the resonant line is varied in accordance with the capacity variations at C. In any resonant system a change in either inductance or capacity changes the resonant frequency of the system. Since the pickup 5, 6 is a capacity device it will change the efiective capacity of the system to which it is coupled. Consequently, the effective length of the system is changed. It is pointed out that the electrical length of an antenna, for example, is affected by the proximity of other objects. Similarly, in the case of Fig. 1, due to the reactance drop across the pickup, any change in the value of C causes a change in its reactance drop. This, in turn, causes a change in the effective length of the resonant line. Y

The coupling condensers I 0 and 9, as stated above, will not prevent changes in capacitance of C from affecting the natural frequency of the resonant line, because the capacities are relatively large and, as a result, at this high frequency the reactances of the coupling condensers I8 and 9 are low. The conductor 7' is preferably loosely coupled to the grounded conductor I9 over most of the length of the latter. Since a standing wave exists on conductor I9, the end away from the grid I3 is hot, and, therefore, it can be coupled to magnetically and electrostatically.

I do not wish to restrict my invention to any specific degree of loose coupling between conductors I and I9. By way of example the coupling ma be chosen from a range of 0.05 to 0.2. The coupling should be sumciently loose to permit changes in the effective electrical length to produce substantial and corresponding changes in the amount of power absorption from the oscillator. Since the resonant line is coupled to the oscillator, any changes in the effective length of the resonant line produced in response to changes at C cause corresponding changes in the amount of power absorption from the oscillator. As the power absorption from the oscillator changes, the space, or plate, current of the oscillator tube varies in accordance with the capacitance variations at pickup condenser C. These current variations cause corresponding voltage variations across resistor I8. The voltage variations correspond to the audio frequency vibrations of stylus 4, and, hence, the audio voltage developed across output resistor I8 is taken off by condenser 22 and applied to the input electrodes of the following audio frequency amplifier which may terminate in any suitable reproducer.

In Fig. 2 I have shown an embodiment of the invention wherein the base of the tone arm functions as a mounting for the oscillator circuit. Additionally, only one of the conductors of the resonant line projects beyond" the tone 5, arm. Further the coupling condenser II] is provided by a pair of concentric metallic rings so as to permit the tone arm to move relatively to the concentric line resonator I5, I9 without any direct connection between the tone arm and fixed line 8.

The tone arm I is supported at the usual swivel joint :2, the support base 30 for the joint 2 being composed of insulation material. It will be understood that the base 30 is hollow and has a circular cross-section. The dependent periphery of base 36 is provided with a metal ring M at the inner face thereof, as shown in Fig. 2. The conductor 8 is connected between fixed electrode 6 and the ring 3i, the conductor 3 passing through a suitable aperture in base 33. The ring 3|, being mixed to base 30, moves with the base, swivel joint 2 and tone arm I as a single unit. Tone arm I, of course, is adapted to move vertically relative to base 30. The base 38 is movably mountedon the upper end of a vertical post 32 whose lower end is rigidly afiixed to a circular mounting plate 33 provided with apertures to receive securing bolts or screws. The plate 33 is aifixed to a portion of a chassis.

The post 32 is provided with a ring 34 of composite material having an outer section 35 made of metal. The periphery of metal section 35 is in alignment with that of ring 3| but is spaced therefrom to provide a relatively large capacity II). The ring section 35 is rigidly affixed to an inner insulation section rigidly affixed to post 32. It will, therefore, be perceived that spaced metallic rings 3i and 33 provide the condenser I which connects tone arm conductor 8 to conductor 8.

The conductor 8 has its upper end secured to ring 35, while its lower section is in proximity to the outer cylindrical metal shell I9 of a concentric line resonator. The latter includes a metallic tube I6 axially located relative to the metal shell I9. Insulating'discs 40 and M act physically to close the ends of the shell I3. Electrically, however, the shell is open at both ends, and permits considerable leakage and consequent radiation. The ends of metal tube IS, the inner conductor of the concentric line resonator, project somewhat beyond the discs 40 and II.

L-shaped support brackets 42, secured to plate 33 and disc 43, retain the concentric line resonator I6, IS in fixed dependent relation from mounting plate 33. The circuit components of the oscillator are connected as indicated in Fig. 2. The miniature type oscillator tube II has its socket pins plugged into suitable socket apertures (not shown) provided in the disc H. The outer shell I9 corresponds to grid conductor I9 of Fig. 1, while the axial metal tube I6 corresponds to plate conductor I of Fig. 1.

The condenser 29 is connected from the shell I9 to the inner conductor I6 adjacent the inner face of disc 4I. Wire 53 connects the inner conductor IE to the late pin of tube II. Resistor I4 is connected from the grid pin to the cathode pin of tube II, and the cathode end of resistor I4 is connected to metallic contact 69 adapted to be connected to the -B (or ground point) terminal of the direct current source (not shown).

The condenser 2| is connected from the shell I9 to the upper extended end of inner conductor I6. The output resistor I8 is connected from the same point on conductor I6 to a metallic contact III on upper disc 40. The +15 terminal is connected to the end of resistor I8 at contact III. Output condenser 22 is connected to the opposite end of output resistor I8. It will, therefore, be perceived thatlthe circuit connections between tube II and concentric line resonator I6, I9 are precisely the same as those shown in Fig. 1.

The conductor 8 is passed through suitable bores in the plate 33 and closure disc 40 to the interior of shell I9. The greatest intensity of field exists inside the shell I9. The wire 8 could be located approximately of an inch from the inner face of shell l9. This spacing is not critical, and anything from to of an inch is suitable. However, the conductor 8 could also be located outside shell I9, if desired, since there is enough leakage outside the shell to permit satisfactory operation.

. It will be understood that the concentric line resonator I6, I9 is constructed and designed to tune to a high frequency, of the order of 200 me. The resonant line I, 88' has an effective electrical length such that it is approximately half a wavelength long at 200 mc. It will be noted that the section I of Fig. 1 has been dispensed with. Actual experimental operation with asystem of the construction of Fig. 2 has shown that it is sufficient to have but one of the resonant lines project from the tone arm in order to secure a substantial Variation in oscillator tube space current. Variation of the capacity of pickup condenser C causes a corresponding variation in efiective electrical length of line 1, 88 thereby resulting in a corresponding variation of the absorption of oscillatory energy. As explained before, there is secured a variation in voltage across resistor I8 which is representative of the audio frequency modulation of the record grooves.

The mounting plate 33 performs a double function in the system. It acts as the base of the tone arm, while concurrently supportin the dependent tank circuit. This construction makes for economy of components, and provides compactness. If it is desired to include the section I, as shown in Fig. 1, it is only necessary to employ a dual spaced ring construction each spaced ring pair being a duplicate of the spaced ring construction of Fig. 2. It is pointed out that at an operating oscillator frequency of 200 mc., the overall physical length of the shell below plate 33 was 3 inches and its diameter 1 inches, while the inner conductor I6 was pipe having an internal diameter of three-eighths of an inch.

These illustrative dimensions show how relatively compact the auxiliary components are in the utilization of my invention with a tone arm construction of the type disclosed in my aforesaid application.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent toone skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In combination with a phonograph tone arm: a condenser pickup held on said arm; a resonant line consisting essentially of a pair of conductors provided with input terminals coupled to said pickup; said conductors running through the tone arm; at least one conductor projecting beyond the tone arm; a high frequency oscillator comprising a concentric line resonator;

said projecting conductor being loosely coupled to the outer shell. of the resonator.

2. In combination, a tone arm, a support plate, means pivotally coupling said tone arm to said plate, said coupling means including a pair of spaced relatively movable metallic ring sections forming a capacitor, a concentric line resonator comprising an outer metal shell and an axial metal conductor, insulator discs closing the ends of the shell, support means securing one disc to said plate thereby to support the resonator therefrom, a variable capacity pickup mounted at the free end of the tone arm, a pair of conductors running the length of the tone arm, said conductors having their outer ends connected to said pickup, one conductor having its inner end connected to one of the two ring sections, a third conductor having one end connected to the other ring section and extending into the interior of said shell, and an oscillator tube mounted on the second insulator disc, and having electrical connections to said shell and axial conductor, to cause said resonator to act as an oscillator tank circuit. 7

3. In combination with a pivotable phonograph tone arm: a variable capacity pickup held on said arm; a resonant line consisting of a pair of conductors provided with outer terminals connected to said pickup, said conductors running through the tone arm; at least one conductor having its inner end projecting beyond the tone arm; pivotal coupling means coupling said projecting conductor to a fixed conductor located in the field of a high frequency oscillator; and said pivotal means consisting of a pair of spaced metallic ring sections, one of which is solidly attached to pivot with the tone arm.

4. In combination with a pivotable phonograph tone arm: a variable capacity pickup held on said arm; a resonant line having two conductors located-within the arm connected to the pickup; pivotal coupling means including a pair of spaced ring sections, one of which ismounted for pivotingwith the tone arm and forming a non-mechanical coupling connecting one. of said line conductors with a fixed conductor external to the arm for absorbing energy from an external electromagnetic field.

5. In a record reproducer system a high frequency oscillator; a phonograph tonearm; a variable capacity pickup device held on the arm; a resonant line having two conductors located within the arm and connected to the pickup; a

non-metallic coupling between one. of said conductors and a conductor external to the arm; said coupling comprising a pair of concentric spaced metal ring sections, one. being relatively fixed and the other being movable with the tone.

arm; means electrically connecting the line to the movable rin section, and means connecting said external conductor to the fixed ring.

6. In combination: high frequency oscillator structure including conductor element for establishing a held of high-frequency electromagnetic energy; translating means in the form of a resonant. line terminating in signal-responsive variable reactance structure and having an effective electrical length'approximating. half the wave length of the high-frequency energy; the resonant line including two elongated conductors each. connected at one end to the variable reactance structure for causing variations in reactance to correspondingly vary the efiective electrical length of the line; one line conductor having a portion extending adjacent to but spaced from the field-establishing conductor elements of the oscillator structure to loosely couple the resonant line to the oscillator structure and cause absorption of the high frequency energy by the resonant line to an extent varying in accordancewith the effective electrical length of the line, and thereby produce field strength changes reflected as corresponding voltage changes in the oscillator structure; and output means connected to the oscillator structure for deriving electrical signals corresponding to said voltage changes.

'7. The combination as defined by claim 6 in which the signal-responsive reactance structure is movable with respect to the oscillator structure, and the line conductor portion extending adjacent to the field-establishing conductor elements is held in substantially constant position relative to said conductor elements and is connected to the signal-responsive reactance structure through a capacitance provided by spaced relatively movable conductive members.

8. A high frequency oscillator system for phonograph record translation and the like comprising in combination, electronic amplification structure provided with input and output electrodes, a tuned high frequency tank circuit coupling said electrodes together to provide oscillations of a predetermined frequency, an electromechanical transducer device provided with a displaceable translating element, a resonant line coupling said transducer device to said tank circuit, said line having one conductor terminating in spaced capacitive coupled relation to an oscillation conducting portion of said tank circuit for causing absorption of oscillatory energy from the oscillator by the line in response to displacements of said translating element and resultant variations in the effective electrical length of the line, andtranslating means connected to said amplification structure to translate the absorption variation into corresponding electrical signals.

CHESTER M. SINNETT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,796,155 Lefiler Mar. 10, 1931 2,095,980 I-Iansell Oct. 19, 1931 2,160,466 Usselman May 30, 1939 2,209,541 Rust July 30, 1940 2,238,117 Koch Apr. 15, 1941 2,243,216 Lindenblad May 27, 1941 2,312,919 Litton Mar. 2, 1943 2,372,701 Antalek Apr. 3, 1945 OTHER REFERENCES Practical Analysis of Ultra High Frequency. pages 5 and 6, published by'RCA Services 00., Camden, New J ersey; 

